Carbon fibers have been widely used as a structural material in the form of a composite thereof with a matrix material such as a resin or a metal. Since carbon fibers have excellent mechanical, thermal, electrical and antimicrobial properties, they are used as reinforcing fibers for structural members of aerospace vehicles such as crafts, rockets, etc., as well as structural members of sporting goods such as golf club shafts, tennis rackets, fishing rods, etc. A generally adopted process for producing such carbon fibers comprises heating acrylic fibers as the raw material (precursor) in an oxidizing atmosphere of about 200.degree. to 300.degree. C. to convert the precursors into oxidized fibers, and subsequently heating the oxidized fibers in an atmosphere of at least about 1,000.degree. C. to carbonize the same.
Investigations have recently been made on the use of carbon fibers in the fields where a higher performance is required, for example, in primary structural members of aircrafts, which use may be attained by further improving the performance, particularly the tensile strength, of carbon fibers while keeping the modulus of elasticity thereof on a high level. Thus, there has been a growing demand for higher quality and performance of carbon fibers. Many proposals have heretofore been made with a view to coping with such demand. However, the proposed processes have achieved an increase or improvement in tensile strength but no increase or improvement in modulus. Most of the proposed processes involve a problem that the tensile strength is lowered when an enhancement of or an improvement in the tensile modulus is intended. For example, as one of those proposals, Japanese patent application Kokai publication No. 55-163217 discloses a process of producing carbon fibers of a high performance which uses an acrylic precursor obtained by a dry-jet wet spinning and a multi-stage drawing. However, this Japanese publication does not disclose oxidation and carbonization steps operated under a very high tension. On the other hand, European application publication No. 0159365 Al discloses oxidation and carbonization steps operated under a very high tension, but does not disclose a dry-jet wet spinning and a multi-stage drawing.